1. Field of the Invention
The present invention relates to a digital circuit for generating a binary signal on the occurrence of a given frequency ratio of two signals. More particularly, the invention relates to a circuit wherein the higher-frequency, first signal is applied to the count input of a first up-counter whose maximum count is greater than the frequency ratio, wherein the second signal is applied to the count input of an up-down counter, and which includes at least one monostable multivibrator.
2. Description of the Prior Art
A digital circuit of this kind is disclosed in U.S. patent application Ser. No. 350,556 filed Feb. 22, 1982, now U.S. Pat. No. 4,454,531. It may be needed in color-television receivers, for example, to determine the ratio between the chrominance-subcarrier frequency and the horizontal frequency, which ratio is given at the transmitting end both in the PAL system and in the NTSC system.
As is well known, in television receivers, not only the chrominance-subcarrier frequency and the horizontal frequency but also the field frequency and the horizontal frequency are characterized by a fixed ratio. The "recognition" signal of the latter frequency ratio can be used to control a vertical portion generating the vertical synchronizing pulses by frequency division of a signal of twice the horizontal frequency in order to achieve improved vertical synchronization. The principles of such vertical portions are disclosed, for example, in Offenlegungsschrift DE No. 24 49 535 Al. In the prior art arrangements, an inaccuracy may occur which is visible on the screen by image motion and is due to insufficient detection of but slight frequency errors. The inventor has found that the image motion can be completely eliminated by generating a signal indicating the presence of the given frequency ratio, and using this signal for control purposes.
While the problem to be solved by the invention was just explained with reference to television receivers, the use of the invention is not limited to this specific case, which only represents a preferred application of the invention.